This invention relates to the manufacture of glass by a process in which molten glass is delivered onto and supported by a pool of molten metal during the formation of flat glass. In particular, this invention relates to improved method and apparatus for circulating molten metal within such a glass-supporting pool of molten metal.
It is known that a layer of molten glass may be continuously delivered onto a supporting pool of molten metal for forming and that the molten glass as initially delivered may be delivered between guides or restraining members which extend longitudinally in a glass forming chamber from the location of molten glass delivery to locations within such a forming chamber downstream from the point of delivery. An apparatus of this kind is shown in U.S. Pat. No. 3,843,346 to Edge et al. The restraining members or guides employed in such a device are generally spaced inwardly from the side walls of the forming chamber so that in the vicinity of the end of the forming chamber where molten glass is delivered to it the supporting pool of molten metal is divided into three portions: a main, central portion between the spaced restraining members upon which a layer of molten glass is directly delivered and two marginal portions of the pool, each of which is between a restraining member and the chamber side wall closest to it. The restraining members may be substantially parallel as illustrated in U.S. Pat. No. 3,843,346 or they may be arranged in a diverging relationship so that they are closer together at the location of molten glass delivery and more widely spaced at their terminal ends downstream from that location (downstream being defined as the direction in which glass is advanced through the forming chamber).
According to the teachings of Dickinson in U.S. Pat. No. 3,647,408, restraining members, which are illustrated in that patent as fenders, may be provided on either side of an advancing glass ribbon and means may be provided for pumping a portion of the molten metal in the glass-supporting pool of molten metal outwardly beneath each of the restraining members at their ends closest to the location at which molten glass is delivered to a forming chamber (that is, at their upstream end) and forcing the molten metal to flow in a generally downstream direction beyond the downstream ends of the restraining members and then inwardly to the central portion of the pool of molten metal. By virtue of a continuity of mass and momentum, the inwardly directed flows of molten metal at the downstream ends of the restraining members merge to establish an upstream directed flow of molten metal beneath the advancing layer of glass and counter to its direction of its advance. According to the teachings of Dickinson, the flow pattern established is useful for controlling thermal conditions within a forming chamber. The apparatus and method of Dickinson contemplates the use of electromagnetic induction motors as means for pumping the molten metal, the use of which devices substantially minimizes or eliminates any significant wave action in the surface of a pool of molten metal. The restraining members employed in the practice of the Dickinson invention are spaced at locations relatively remote from the location of molten glass delivery and are positioned at locations which permit and possibly require the unhindered spreading of a delivered layer of molten glass prior to encountering the described restraining members. U.S. Pat. No. 3,479,171 to Robinson et al describes the establishment of flows in a glass-supporting pool of molten metal which are substantially identical to the flows established according to the teachings of Dickinson. However, no longitudinal glass restraining members are disclosed to act as barriers between the marginally disposed, downstream directed flows and the centrally disposed, upstream directed flows within the pool of molten metal. Short transverse barriers are disclosed by Robinson et al as means for insuring that the outwardly directed flows of molten metal in the upstream portion of the forming chamber are redirected along the marginal portions of the chamber in a downstream direction.
U.S. Pat. No. 3,607,203 to Fujimoto et al discloses an apparatus and method for establishing flows about restraining members wherein the flows are in the same downstream direction as the advance of glass in the central portion of the forming chamber and then in an upstream direction in the marginal portions between the restraining members and the side walls of the forming chamber with resulting inwardly directed flows in that portion of the pool of molten metal closest to the location at which molten glass is delivered onto the pool of molten metal. This patent illustrates a puddle roller which may be positioned in a marginal space between a restraining member and a side wall of the forming chamber in order to control the flow of molten metal within the marginal portion of the pool. While the patent of Fujimoto et al recognizes the possibility of directing the flow of molten metal in either direction, there is no isolation of the molten metal at its surface between the three portions of the pool and therefore no arrangement for causing surface waves in the molten metal in the marginal portions alone.
In another environment of a float forming process and, in particular, in the downstream end portions of a float forming chamber, there may be provided means for removing dross from the surface of the glass-supporting pool of molten metal. Skimming devices are commonly provided in U.S. Pat. No. 3,653,869 to Biagini and illustrates a rotor that is provided with lands, which dip into the surface of molten metal as the rotor is rotated, and propels dross along the surface of the pool of molten metal. Since these lands dip into the molten metal, they may, if properly sized, establish a wave action in the surface of the pool of molten metal. The rotor of Biagini may be construed as an impeller for moving dross and molten metal and, thus, causing a flow of molten metal.
The patent to Fujimoto et al, in particular, is concerned with problems of impurities in the glass-supporting molten metal which may be the source of bottom surface defects in the glass produced, yet the methods and apparatus described in the patents mentioned above are not directed to any particular arrangements which would enhance the purification of the molten metal by the reducing atmosphere maintained in the space above the molten metal within a glass forming chamber. The present invention is directed to such a method and apparatus.